1. Field of the Invention
The present invention relates to a solid state image sensor drive apparatus and particularly to a drive apparatus which controls the operation of a solid state image sensor such as a CCD in accordance with the brightness of an object to be imaged.
2. Description of the Prior Art
A focus detecting apparatus of a camera is known as an example of a control apparatus using a CCD. Such a focus detecting apparatus evaluates or calculates a focus condition or an in-focus position of an objective lens by analyzing an image signal read out from CCD based on an image of an object to be photographed formed on the CCD cell through the objective lens, so that the objective lens is brought into an in-focus position in accordance with the result of the analysis. In such a focus detecting apparatus, the position of the objective lens to be driven is fixedly determined, in response to an operation for the start of exposure operation, by definitely adopting the result of the analysis of the image signal provided immediately before the start of the exposure operation. However, prior to that stage, that is, when a shutter release button is pressed at a first stage, an in-focus position is repeatedly detected and the objective lens is successively moved to an in-focus position accompanied by movements of an object and the camera.
More specifically, reading of an image signal from the CCD and analysis of the image signal are repeated at high speed so that an in-focus position is evaluated according to movements of the object and the camera. A solid state image sensor such as a CCD receives a light image impinged on a photodiode array portion thereof and converts the amount of received light into an electric charge by individually integrating photoelectric current generated in each photodiode of the photodiode array so that the accumulated electric charge is read out as a voltage signal.
The above described focus detecting apparatus is arranged to output an image signal of a level enough for the focus detection by changing, in accordance with brightness of an object, a period of time for integrating photoelectric current of the CCD so as to ensure reliability of focus detecting operation. As a result, in the above described focus detecting apparatus, an integration stop signal for designating the termination of the integration or accumulation of the CCD is outputted non periodically and therefore it is asynchronous with respect to the control operation of a control device such as a microcomputer which controls various operations including image signal reading operation of the focus detecting apparatus. Consequently, the following problem is caused.
In the CCD, photoelectric current generated in each photodiode of the photodiode array is applied for the current integration or charge accumulation by means of a capacitor component of each photodiode and a voltage signal representing the accumulated charge as an image signal of each capacitor component is moved or shifted in parallel to a shift register by a shift pulse so that the content of the shift register is serially read out by transfer clock signals. Accordingly, the above described shift pulse is outputted with timing dependent on the brightness of an object. Since the shift pulse is outputted asynchronously with respect to the operation of the microcomputer, if the shift pulse is outputted while the microcomputer performs other control operation, the image signal is unavoidably transmitted successively from the CCD.
As a result, since the microcomputer can not store or read the image signal in a memory or a RAM thereof, the accumulation operation of the CCD is wasted. On the contrary, if the microcomputer is adapted to interrupt other processing operation to receive the image signal transmitted from the CCD when a shift pulse is outputted, a delay is caused in other control operation by the microcomputer although the operation of the CCD is not wasted, which makes it difficult for the apparatus to perform processing operation at high speed.